In the field of digital imaging a wide variety of image acquisition techniques have been developed that render a digital representation of a medical image.
Among such techniques are computerised tomography, nuclear magnetic resonance, ultrasound, detection of radiation by means of a CCD sensor or a video camera, radiographic film scanning, direct radiography techniques using radiation sensitive detector elements etc.
In still another technique a radiation image, e.g. an X-ray image of an exposed object (such as the body of a patient) is stored in a photostimulable phosphor screen. Such a photostimulable phosphor screen comprises a layer of photostimulable luminescent material and a binder. The photostimulable luminescent material is for example a europium-activated barium fluoro-halide phosphor or a Cs-phosphor.
The phosphor is characterised in that it can be energised to an excited state by X-rays and can then be stimulated by light within a first wavelength range to return to a ground state by emission of light within a second wavelength range.
In order to read the image that has been stored in an exposed screen, the screen is scanned two dimensionally with stimulating light.
Light emitted upon stimulation is guided to an opto-electric transducer (or transducer array) that converts the detected light into a digital image representation.
Systems exist that provide enhanced image quality by double sided read out of an exposed screen.
In particular in the field of medical imaging where a radiation image of a patient's body is used by a physician to make a diagnosis, it is extremely important that all imaging defects which may affect the diagnosis are detected and corrected.
In the state of the art a method is known to correct a radiation image read out of a photostimulable phosphor screen for non-uniformity of the detector.
According to this method, a flat field exposure of the detector is performed prior to exposure and read out of a diagnostic image. The image corresponding with the flat field exposure is read out and stored.
When a diagnostic image is read out, this image is also stored. Next, in order to perform correction, both images are retrieved for memory and each of the pixels of the diagnostic image is corrected by means of the corresponding (in register) pixel values of the image corresponding with the flat field exposure.
In the case of double sided read out, this procedure can be applied to the signal read out of each side of the detector. Each of the ‘partial image representations’ read out of one side of the detector is corrected with a correction signal representing an image obtained by flat field exposure of the relevant side of the detector.
Since in either of the cases this correction consists a pixel-by-pixel correction of at least two images that need to be retrieved from memory(pixel-wise division or multiplication of two images per side), the operation is time-consuming and may negatively affect the throughput of the system.
For dual side reading this decrease of the throughput is even twice as important.
It is an aspect of the present invention to overcome the prior art disadvantages.